Im probably heading down to TJ this weekend after a nearly 3 year absence (marriage!). Never been on Cinco de Mayo, will it be a good time to goto Adelitas or empty? I imagine it would be busy but you never know since it is a mexican holiday.

Cinco de Mayo is a holiday promoted by Beer companies in the States.  Not celebrated here.

Although it is an offical Mexican holiday, Cinco de Mayo is not celebrated as much in Mexico as it is in the U.S.  Contrary to what many people think, it is not Mexico's Independence Day.  Rather it commemorates the victory of the outnumbered Mexican forces over the French army at The Battle of Puebla on May 5, 1862.

The battle at Puebla occurred during a chaotic time in Mexico's history.  Mexico gained independence from Spain in 1821 after a long, difficult, and bloody struggle.  A number of internal political takeovers and wars, including the Mexican-American War (1846-1848) and the Mexican Civil War of 1858, had ruined the Mexican economy.  During this period Mexico accumulated heavy debts to several nations, including Spain, England and France, who were demanding repayment. (Similar debt to the U.S. was previously settled after the Mexican-American War.)  France, under Napoleon III, was eager to add to its empire at that time, and used the debt issue to move forward with it goal of establishing control over Mexico. Realizing France's intent of empire expansion, Spain and England withdrew their support. When Mexico finally stopped making any loan payments, France took action on it's own to install Napoleon's relative, Archduke Maximilian of Austria, as ruler of Mexico.  

This resulted in a war with Mexican forces ultimately driving out the French.  The United States was involved in its own Civil War at the time and was unable to provide any direct assistance. While unable to send armed forces to assist Mexico, Lincoln did manage to send arms to Mexico during its struggle against the French, for which he is honored in Mexico. For example, on the Paseo de los Hereos in the Zona Rio of Tijuana is a statue of Lincoln with broken chains in his hands. The chains do not represent the freeing of U.S. slaves, rather they represent the freeing of Mexico from French occupation under Maximilian made possible by Lincoln providing aid to Mexico during its struggle against the French forces of Maximilian.

General Ignacio Seguín Zaragoza, who commanded the victorious Mexican troops at the Battle of Puebla on May 5, 1862, was born in 1829 in Goliad, Texas (when Texas was still part of Mexico.) Zaragoza’s favorite son status among Mexican-Americans in Texas resulted in large Cinco de Mayo celebrations being held in Texas. These celebrations gradually spread thought other parts of United States until today Cinco de Mayo is more vigorously celebrated in the United States (especially in cities with large Mexican-American populations) than in most parts of Mexico.

So since Americans tend to celebrate it, the question would still remain... will TJ be full of Americans getting borracho y loco?

Basically it will be business as usual.  Probably there will be a more gringos than normal, so expect longer than average lines (especially for cars) when returning.